Among fuel gases, particularly, natural gas (a gas mixture mainly consisting of methane gas, propane gas, or the like), when in a form of liquefied natural gas, has a volume reduced down to 1/600th of that in its gaseous state. Accordingly, natural gas is transported in the form of liquefied natural gas (hereinafter, LNG) from a producing area to a consuming area or other areas. An LNG carrier equipped with a tank covered and surrounded by a heat-insulating material is used for the transportation.
However, aforementioned LNG has an extremely low boiling point of −162° C., and has a characteristic that LNG rapidly evaporates as the temperature increases. Thus, it is necessary to keep LNG under the extremely low temperature condition during transportation. For this reason, a dedicated container having a great cold-reserving ability is required therefore.
In recent years, as a form of fuel gas, attention has been paid to a gas hydrate that can be transported stably at a milder cooling temperature than that for above-described LNG. This gas hydrate is formed as follows. Specifically, raw material gas such as natural gas and raw material water are brought into gas-liquid contact at a temperature of approximately 0 to 5° C. under a high atmospheric pressure of approximately 3 to 5 MPa. Then, hydration reaction takes place to form a gas hydrate. In the gas hydrate, molecules of natural gas or the like are trapped in a lattice formed of aggregated multiple water molecules.
In order to keep a gas hydrate stable under atmospheric pressure, the gas hydrate needs to be stored at approximately −80° C. or below under equilibrium. Meanwhile, the gas hydrate has a property unique to hydrates, i.e., a so-called “self-preservation effect” that the gas hydrate is relatively stable at a temperature around −20° C. which is higher than the equilibrium temperature. Because of this self-preservation effect, the gas hydrate has a superb characteristic that the gas hydrate can be stored or transported over an extended period under a far milder atmosphere than that for LNG, i.e., at approximately −20° C. to −10° C. under atmospheric pressure.
Furthermore, for example, a natural gas hydrate (hereinafter, NGH), when in a form of NGH, has a volume approximately 1/170th of that in its gaseous form. Although having a volume reduction ratio lower than that of LNG, NGH does not need to be kept at such an extremely low temperature of −162° C. as in the case of LNG. Moreover, the NGH can be stored or transported relatively stably over an extended period under atmospheric pressure. Furthermore, the NGH does not require a storage container as highly durable and highly heat-insulating as that for LGN. Thus, a transport ship, cargo ship, and the like can be utilized after being reconstructed for NGH transportation, saving the cost for constructing a dedicated ship therefore, and the like.
On the other hand, a gas hydrate such as NGH is formed in a powder snow-like form, and accordingly has problems of a low bulk density and also a poor handling property. For this reason, as a way of decreasing the surface area and also increasing the bulk density, such a gas hydrate is compression-molded into a shape of almond, lens, sphere, or the like. However, a gas hydrate pellet obtained by compression molding as described above has an improved decomposition resistance. For this reason, methods for efficiently decomposing and gasifying such a gas hydrate have been proposed.
[In-Water Stirring Method]
FIG. 6 shows a scheme of a continuous introduction-type gassifier 41 (see, for example, Patent Document 1). Pellets 31 are sequentially introduced into a container 11 through a supply inlet 12, and brought into contact with water 32 whose temperature has kept at 1° C. to 5° C. to decompose the pellets for gasification. Moreover, the continuous introduction-type gasifier 41 includes a heater 18 to maintain the aforementioned temperature by heating the water 32, since the introduced pallets 31 are normally around −25° C. to −5° C. Furthermore, a stirrer 42 is provided to stir the water 32 in the container 11 to increase the contact efficiency between the water 32 and the pellets 31 so that the heat can be rapidly transferred therebetween, and that the decomposition of the pellets 31 can be accelerated. Additionally, a discharge pipe 14 is provided to maintain the water level in the container 11 at a predetermined height, since the gas hydrate includes hydration water contained at the time of hydration reaction with raw material water and releases the water and a gas upon decomposition.
Problems of the stirring method are that a crushing and stirring unit is needed, and that additional power consumption is required. Moreover, for stirring, a large amount of water must be present in a space around pellets, and accordingly the size of the gasification tank tends to be large.
In addition, a transfer installation is needed for transferring the pellets 31 from a storage·transportation container thereof to the gasifier 41, increasing the size of the entire gasifier 41 facility.
[Water Spraying Method]
FIG. 7 shows a gasifier 43 (see, for example, Patent Document 2.) allowing the transportation and gasification of pellets 31. In the gasifier 43, water 32 is sprayed onto the gas hydrate pellets 31 stored in a container 11.
This gasifier 43 enables the storage·transportation and gasification of the pellets 31 to take place in the same apparatus, making the whole apparatus compact. Meanwhile, as the gasification proceeds, the amount of the pellets 31 filled in the container 11 is successively decreased, leading to a problem that the amount of gas generated is likely to vary. This makes it difficult to achieve stable gas supply.
Moreover, there is another problem that, even when the spraying of the water 32 is stopped, the decomposition of the pellets 31 cannot be stopped, since the sprayed water 32 reaches all over the pellets 31 filled in the container 11.
[Immersing Method]
FIG. 8 shows a gasifier 45 (see, for example, Patent Document 3.) allowing the transportation and gasification of pellets 31. The pellets 31 are filled into a container 11 and transported. The pellets 31 are decomposed by introducing water 32 into the container 11. This gasifier 45 is designed so that the water 32 will be introduced from a bottom portion of the container 11. By controlling the water level in the container 11, the amount of the pellets 31 immersed in the water 32 is adjusted. Furthermore, the amount of gas generated is controlled by the temperature and the amount of water introduced.
The pellets 31 located on the bottom portion side of the container 11 are immersed in the water 32, whereas the pellets 31 located at the middle portion and top portion of the container 11 never comes into contact with the water 32. Thereby, the amount of gas generated can be accurately controlled by the adjustment of the water level. Thus, a gas can be supplied stably to the outside.
However, in this gasifier 45, when the pellets 31 on the bottom portion side of the container 11 are decomposed, a cavity is formed as shown in FIG. 5. As a result, there is a problem that the pellets 31 at the other portions are not gasified due to a so-called “bridge phenomenon” in which the pellets 31 are not supplied downward any more.
This bridge phenomenon occurs because the pellets 31 that are in contact with an inner wall of the container 11 adhere to the inner wall, and because the pellets 31 are supported at the wall surface by receiving a compression force of their own weights.
In order to eliminate a bridge 33 formed in the container 11, some counter-measure needs to be taken such as provision of breaking means for physically breaking the bridge 33 in the container 11. Such installation of a mechanism such as a hammer for breaking the bridge 33 in the container 11 means a less loading space for the pellets 31.
Meanwhile, in a case where the breaking means is not installed in the container 11, the container 11 has to be opened every time the bridge 33 is to be broken. As a result, there are problems that a gas is escaped concurrently with the opening of the container 11, and that the opening operation is labor consuming.    Patent Document 1: Japanese patent application Kokai publication No. 2004-75849    Patent Document 2: Japanese patent application Kokai publication No. 2006-160841    Patent Document 3: Japanese patent application Kokai publication No. 2006-138349